The present invention relates to power electronics equipment. Specifically, the present invention relates to power electronics equipment well suited for transmitting signals to switching devices via air-cored insulating transformers.
On-vehicle equipment mounts a step-up and step-down converter and an inverter on the driving system of a motor that generates driving power for improving conversion efficiency and for reducing the energy consumption.
FIG. 11 is a block diagram schematically showing a vehicle driving system that employs a conventional step-up and step-down converter.
Referring now to FIG. 11, the vehicle driving system includes a power supply 101 that feeds electric power to a step-up and step-down converter 102 that boosts and steps down a voltage, an inverter 103 that converts the voltage outputted from step-up and step-down converter 102 to the components of a three-phase voltage, and a motor 104 that drives the vehicle. Power supply 101 may comprise a voltage fed through overhead wires or batteries connected in series.
In driving the vehicle, step-up and step-down converter 102 boosts the voltage of power supply 101 (e.g. 280 V) to a voltage suited for driving motor 104 (e.g. 750 V) and feeds the boosted voltage to inverter 103. By controlling the ON and OFF of the switching devices in inverter 103, the voltage boosted by step-up and step-down converter 102 is converted to the components of a three-phase voltage and a current for each phase of motor 104 is made to flow. By controlling the switching frequency of inverter 103, the vehicle speed can be changed.
In braking the vehicle, inverter 103 controls the ON and OFF state of the switching devices, synchronizing the voltage components generated in the phases of motor 104 to conduct rectifying operations for converting the three-phase voltage to a DC voltage. The DC voltage feeds step-up and step-down converter 102. Step-up and step-down converter 102 steps down the voltage generated in motor 104 (e.g. 750 V) to supply the voltage of power supply 101 (e.g. 280 V) in order to conduct regeneration operations.
FIG. 12 is a block circuit diagram of the step-up and step-down converter shown in FIG. 11.
Referring now to FIG. 12, step-up and step-down converter 102 includes a reactor L for energy storage, capacitor C that accumulates electric charges, switching devices SW1 and SW2 that make a current flow into inverter 103 and interrupt the current flowing into inverter 103, and control circuits 111 and 112 that generate control signals directing the conduction and non-conduction of switching devices SW1 and SW2.
Switching devices SW1 and SW2 are connected in series. Power supply 101 is connected to the connection point of switching devices SW1 and SW2 via reactor L. In switching device SW1, an insulated gate bipolar transistor (hereinafter referred to as an “IGBT”) 105 that conducts switching operations in response to the control signal from control circuit 111 is disposed. A free-wheel diode D1, which makes a current flow in the direction opposite to the flow direction of the current flowing through IGBT 105, is connected in parallel to IGBT 105.
In switching device SW2, an IGBT 106 that conducts switching operations in response to the control signal from control circuit 112 is disposed. A free-wheel diode D2, which makes a current flow in the direction opposite to the flow direction of the current flowing through IGBT 106, is connected in parallel to IGBT 106. The collector of IGBT 106 is connected to capacitor C and inverter 103.
FIG. 13 is a wave chart describing the waveform of the current flowing through reactor L shown in FIG. 12 in the boosting operation.
Referring now to FIG. 13, as IGBT 105 in switching device SW1 turns ON (conductive) in the boosting operation, a current I flows through reactor L via IGBT 105, storing the energy of LI2/2 in reactor L.
Then, as IGBT 105 in switching device SW1 turns OFF (becomes nonconductive), a current flows through free-wheel diode D2 in switching device SW2, transferring the energy stored in reactor L to capacitor C.
In the stepping down operation, as IGBT 106 in switching device SW2 turns ON (becomes conductive), a current I flows through reactor L via IGBT 106, storing the energy of LI2/2 in reactor L.
Then, as IGBT 106 in switching device SW2 turns OFF (becomes nonconductive), a current flows through free-wheel diode D1 in switching device SW1, regenerating the energy stored in reactor L to power supply 101.
By changing the ON-period (ON duty) of the switching devices, the boosted and stepped down voltages may be adjusted. The approximate voltage value is obtained from the following formula (1).VL/VH=ON duty (%)  (1)
Here, VL is the power supply voltage, VH is the voltage after the boosting or the stepping down, and the ON duty is the ratio of the conduction period of switching device SW1 or SW2 to the switching period thereof.
Since variations are caused in the load and the power supply voltage VL in practice, the ON period (ON duty) of switching devices SW1 or SW2 is controlled by means of monitoring the voltage VH after the boosting or the stepping down so that the voltage VH after the boosting or the stepping down may be equal to the reference value.
Since control circuits 111 and 112 grounded to the vehicle body are on the low voltage side, the arms connected to switching devices SW1 and SW2 are on the high voltage side. So as not to expose any human body to danger even if an accident such as the breakdown of switching device SW1 or SW2 occurs, signal transmission and reception are conducted between the arms and control circuits 111, 112 via insulating transformers, while the arms and control circuits 111, 112 are electrically insulated from each other by the insulating transformers.
FIG. 14 is a top plan view schematically showing a conventional insulating transformer for signal transmission. Referring now to FIG. 14, the insulating transformer includes a magnetic core MC. A primary winding M1 and a secondary winding M2 are wound around magnetic core MC. Magnetic core MC may be made of ferrite, permalloy, and similar ferromagnetic material. The magnetic flux φ generated by the current fed to primary winding M1 is localized into magnetic core MC and made to pass through magnetic core MC. Magnetic flux intersects secondary winding M2, generating a voltage dφ/dT across secondary winding M2. Since a closed magnetic path is formed by using magnetic core MC, the adverse effects of the external magnetic field are reduced and the coupling coefficient of primary winding M1 and secondary winding M2 is increased.
FIG. 15 is a block diagram of a signal transmission circuit using a conventional insulating transformer for signal transmission.
Referring now to FIG. 15, a first end of the primary winding in an insulating transformer T is connected to the drain of a field effect transistor M1 via a resistor R1, and a first end of the secondary winding in insulating transformer T is connected to a demodulator circuit 203. A local oscillation signal generated in a local oscillator circuit 201 is inputted to a modulator circuit 202. As a PWM signal SP is inputted to modulator circuit 202, the local oscillation signal is modulated by PWM signal SP and the modulated local oscillation signal is inputted to the gate of field effect transistor M1 for the control signal thereof. As the control signal is inputted to the gate of field effect transistor M1, a modulated signal modulated at a high frequency, is inputted to demodulator circuit 203 via insulating transformer T and PWM signal SP is demodulated in demodulator circuit 203.
Patent Document 1 discloses a method of transmitting NRZ (Non Return Zero) data signals through an interface comprised of an isolation barrier and arranged between a first device and a second device connected via a bus to each other, wherein a pulse transformer is employed for the isolation barrier.
Patent Document 2 discloses the connection of a driver formed on a first substrate and a receiver formed on a second substrate by the magnetic coupling using coils.
Patent Document 3 discloses the use of a link coupling transformer as a logic separation circuit for isolating an input circuit and an output circuit from each other.
[Patent Document 1] Japanese Patent 3399950 (Counterpart U.S. Pat. No. 5,384,808)
[Patent Document 2] Published Japanese Translation of PCT International Publication for Patent Application 2001-521160 (Counterpart U.S. Pat. No. 6,054,780)
[Patent Document 3] Published Japanese Translation of PCT International Publication for Patent Application 2001-513276 (Counterpart U.S. Pat. No. 5,952,849)
However, the use of a cored transformer as an insulating transformer for signal transmission is adversely affected by the temperature dependence of the magnetic permeability of a magnetic core material, the high temperature dependence of the coupling coefficient, and the difficulties in reducing the costs and dimensions of the apparatus. Since it is impossible to directly send the PWM signal via the cored transformer, it is necessary to demodulate the signal, modulated at a high frequency, after the modulated signal is received by the secondary winding. Therefore, the circuit scale is inevitably large.
Since the use of an air-cored transformer as an insulating transformer for signal transmission does not employ any magnetic core, the use of the air-cored transformer facilitates reducing the costs and dimensions of the apparatus. However, since the magnetic circuit is not closed, external magnetic fluxes are liable to be superimposed onto the secondary winding as noises, causing malfunctions.
In view of the foregoing, it would be desirable to provide power electronic equipment that facilitates reducing the temperature dependence of the coupling coefficient, reducing the adverse effects of the noises caused by the external magnetic fluxes, and transmitting and receiving signals between the high and low voltage sides while insulating the high and low voltage sides electrically from each other.
Further objects and advantages of the invention will be apparent from the following description of the invention.